岩土工程技术

Application of Brillouin Scattering-based Distributed Optical Fiber Sensing Technology on Pile Foundation Test

Huang Xiaowei, Zheng Jianguo, Yu Yongtang, Liu Zhenghong, Zhang Long

2021, 35(5): 281-285, 293. doi: 10.3969/j.issn.1007-2993.2021.05.001

[Abstract](211) [FullText HTML] (44) [PDF 777KB](52)

Abstract:
Recent advances of BOTDR distributed sensing fiber in pile foundation test are summarized, including the principle, equipment, installation and implementation, data collecting and processing, and results analysis of BOTDR. The test technique and procedure of the most frequently used prefabricated and bored piles are compared and analyzed, as well as the results acquired. Combined with engineering application, a brief comment is made, and further research topics are suggested.

Characteristics of Rock Joints and Fissures Based on Borehole Television Technology

Wang Xiaobing, Wang Junqing

2021, 35(5): 286-293. doi: 10.3969/j.issn.1007-2993.2021.05.002

[Abstract](175) [FullText HTML] (68) [PDF 1420KB](62)

Abstract:
The analysis of rock joints and fissures is an important basis for engineering geological evaluation of rock mass characteristics. Panoramic borehole television technology can clearly and intuitively observe the development characteristics of rock joints and fissures in the borehole. based on the panoramic borehole television image, through the interpretation, statistics and other quantitative analysis of the joints and fissures in two boreholes of a nuclear power project in Hainan, the strike and dip range of the main joints in the rock mass are determined, and the dominant joints are analyzed; the dip angle and linear density of the joints and fissures in the rock mass are statistically analyzed. On this basis, IRMI index of rock mass quality is used to evaluate the rock integrity of the test holes, and the RQD value and acoustic test results are compared. The limitations of panoramic borehole television technology is analyzed. The results show that: the IRMI value of rock mass quality index of borehole television is higher than that of drilling RQD, and the quality degree of rock mass is better than RQD; the structural position, occurrence and filling material of rock mass can be observed directly and clearly through borehole television, and the mechanical fracture and joint fracture of rock mass can be distinguished, which can make up for the lack of core drilling technology to a certain extent. It is more accurate to evaluate the quality of rock mass by combining IRMI index, RQD value and acoustic test results.

Differential Settlement Control of Road Bridge Transition Section in Deep Soft Soil Area

He Changming, Peng Gongxun

2021, 35(5): 294-298. doi: 10.3969/j.issn.1007-2993.2021.05.003

[Abstract](210) [FullText HTML] (34) [PDF 906KB](41)

Abstract:
The differential settlement of road bridge transition section seriously affects the safety and comfort of driving. According to the characteristics of deep soft soil area, such as large depth of soft foundation, high difficulty of soft foundation treatment and high cost, three differential settlement control schemes of rigid transition, flexible transition and rigid flexible combination transition are summarized. According to the three transition schemes, prestressed pipe pile composite foundation, vacuum preloading + air bubble mixed light soil embankment and long short pile composite foundation are respectively adopted for engineering practice. Combined with construction parameters and monitoring data, the reinforcement effect is analyzed and studied, and the technical and economic comparison of three transition schemes for soft foundation treatment is made. The results show that the three transition schemes are effective and feasible, but there are some differences in construction period, settlement control effect and economy. In general, when the filling height is fixed, the cost of rigid transition and rigid flexible transition is relatively high, and the greater the treatment depth, the more obvious the advantage of flexible transition in cost.

Application and Analysis of Bagged Sand Well in Soft Soil Foundation Treatment of Highway

He Luhao, Liu Hongjun, Liu Kelun, He Chengling, Lei Shiwang

2021, 35(5): 299-305, 311. doi: 10.3969/j.issn.1007-2993.2021.05.004

[Abstract](88) [FullText HTML] (39) [PDF 1504KB](24)

Abstract:
Based on a highway project in Foshan, Guangdong Province, the in-situ monitoring data with bagged sand well surcharge precompression method were aquired from heap load to dead load, and the variation of surface settlement, stratified settlement, deep lateral displacement and pore water pressure in the soft foundation section were analyzed. The treatment effect of this method combined with the calculated post-construction settlement was also evaluated. The results show that the maximum settlement of the surface is 291 mm when the soft foundation loaded by 210 days, and the maximum settlement rate of the surface is 8.67 mm/d, which is less than the 10 mm/d limited by the standard. The stratified settlement of soft soil mainly occurs in the soil layer with a depth of 2～6 m, and the lateral horizontal displacement of the soft soil mainly occurs in the soil layer with a depth of more than 6 m, accounting for about 95% of the total displacement. In the whole process of filling preloading, the maximum comprehensive pore pressure coefficient B is 0.371, which is smaller than the 0.4 limited by the standard. The final settlement and post-construction settlement of soft soil subgrade are calculated by hyperbolic method. The post-construction settlement is controlled between 34～64 mm, which is less than the 200 mm limited by the standard. The research results can provide the basis for the design and construction of highway soft soil foundation in this area.

Pumping Test and Numerical Simulation in an Excavation Engineering

Zhang Xing

2021, 35(5): 306-311. doi: 10.3969/j.issn.1007-2993.2021.05.005

[Abstract](221) [FullText HTML] (56) [PDF 4174KB](83)

Abstract:
The design scheme of a project in Yichun, Jiangxi Province involved karst landform and ancient river and did not fully consider the impact of groundwater. The large-scale settlement and cracks were begot around the excavation which accompanied construction dewatering. The foundation pit was forced to backfill above the groundwater level. In order to ensure the safety of the surrounding environment of the foundation pit during the secondary excavation, it was necessary to re-formulate the groundwater control scheme. The foundation pit dewatering numerical simulation was carried out by using the field pumping test data, geological survey data and Midas GTS finite element numerical analysis. The final groundwater control scheme was determined after the comparison of safety and economy. The excavation engineering should considered the groundwater conditions and the surrounding environment, and could be tested by the numerical simulation. Combining pumping test and numerical simulation, different groundwater control schemes were analyzed and reasonable design scheme was proposed. This method has certain reference significance to the foundation pit dewatering project under the complex hydrogeological condition.

Liquefaction Discrimination of Clayed Sand in Certain Field

Li Bing, Guo Hongyun, Wang Xinbo, Sun Chonghua

2021, 35(5): 312-317. doi: 10.3969/j.issn.1007-2993.2021.05.006

[Abstract](325) [FullText HTML] (33) [PDF 728KB](43)

Abstract:
Through laboratory tests, the physical and mechanical properties of clayey sand were analyzed in detail. The standard penetration test, shear wave velocity test, flat dilatometer test and dynamic triaxial test were conducted to identify the liquefaction of clayed sand. The reasons for the difference of liquefaction identification results of various liquefaction discrimination methods are analyzed, and a suitable method for liquefaction identification of clayed sand is proposed. The comprehensive liquefaction discrimination of clayey sand in the field is carried out.

Research on Characteristics of Ground Temperature Variation in Tianjin

Wen Weiguang, Lu Qing, Wang Hui, Liu Xiaolei

2021, 35(5): 318-322. doi: 10.3969/j.issn.1007-2993.2021.05.007

[Abstract](150) [FullText HTML] (32) [PDF 804KB](31)

Abstract:
In order to meet the needs of underground space development and construction in Tianjin, the changes of ground temperature were analyzed and studied based on 38 sets of ground temperature test data of the subway project, and the law of ground temperature changes with environmental temperature and stratum burial depth were found out. The method for determining the depth of the constant temperature layer in this area and the fitting calculation formula of the constant temperature are given. The buried depth and temperature of the constant temperature layer in the central city, the four districts around the city and the Binhai New Area are further studied. The results show that when the ground temperature in Tianjin is stable, the average depth is 8～10 m, and the stable temperature is generally 13～17 ℃. The ground temperature varies drastically with depth in the range of 0～10 m, while it is relatively stable in the range of 10～25 m. The depth of the constant temperature layer in Tianjin can be determined by the minimum depth corresponding to the temperature change rate decreasing to 0.2 or below on the relationship curve between ground temperature and depth, and the constant temperature can be determined by formula calculation based on the average temperature and monthly average temperature. The buried depth of the constant temperature layer in each district is generally about 10 m on average, and the average constant temperature is 16 ℃.

Application of Empty Box Load-reducing Retaining Wall in Deep Filled Foundation

Ge Fan

2021, 35(5): 323-326, 331. doi: 10.3969/j.issn.1007-2993.2021.05.008

[Abstract](172) [FullText HTML] (84) [PDF 797KB](39)

Abstract:
The normal box-type retaining walls are mostly gravity retaining walls formed by filling in the box-type concrete structure, used in road engineering; while the empty box-type retaining walls are mostly used in hydraulic structures. According to the conditions of low foundation bearing capacity and limited space, the design of empty box-type retaining wall can meet the engineering requirements of the project, which not only meets the needs of basement covering and retaining soil, but also meets the requirements of foundation bearing capacity. The economic effect is obvious. A reasonable calculation method is chosen for this unconventional structure. The conventional gravity retaining wall stability checking method is used to check the anti-sliding stability, anti-overturning stability and foundation bearing capacity of the retaining wall, and then the finite element modeling is used to simulate the process of structure construction and filling, analyze the most unfavorable internal force condition of complex box structure, and carry out reinforcement calculation. The calculation principle is clear and the results are accurate, which can be used as a reference for the calculation method of complex retaining structure.

Construction of Bored Pile in Hourglass Karst Collapse Area

Liu Biyan

2021, 35(5): 327-331. doi: 10.3969/j.issn.1007-2993.2021.05.009

[Abstract](134) [FullText HTML] (41) [PDF 712KB](40)

Abstract:
Conventional pile foundation construction technology cannot be used in karst collapse area, and effective construction measures should be taken to ensure safety and quality. Based on the causes and mechanism of hourglass karst collapse, the pretreatment measures before karst collapse and technical measures for pile foundation construction after karst collapse were summarizes combining with engineering examples. The distribution of karst was found out through effective investigation means before construction, and grouting method was adopted for pretreatment. After the collapse, in order to prevent the accident during pile foundation construction caused by the secondary collapse, the required steel platform was erected. The key points of full rotation and full set of construction technology are introduced.

Application of High Bearing Capacity CFG Pile Composite Foundation

Wu Qingsong, Jiang Dawei, Yu Mingbo, Zhang Jiawen, Li Qiming

2021, 35(5): 332-335. doi: 10.3969/j.issn.1007-2993.2021.05.010

[Abstract](168) [FullText HTML] (39) [PDF 883KB](66)

Abstract:
The characteristic value of bearing capacity reaches 620～650 kPa in a super-high-rise building in a residential area in Nanning, Guangxi. Based on the application of CFG pile composite foundation, the single pile reinforcement Q-s curve, composite foundation P-s curve and building settlement observation data of the four high-rise buildings in the project are analyzed. The analysis results show that it is feasible to apply high-capacity CFG pile composite foundation for high-rise buildings in this area. The CFG pile of this project uses rock as the bearing layer of the pile end, which has a high bearing capacity. The thickness of the cushion layer should be adjusted appropriately during design and construction to give full play to the advantages of single pile high bearing capacity. The Geotechnical Engineering Investigation Standard for High-Rise Buildings uses the measured composite foundation deformation modulus to calculate the settlement and the results are relatively close to the actual, but there are still some deficiencies.

Study on the Influence of Shield Synchronous Grouting Slurry Performance and the Optimization of its Proportion

Yang Xing, Zhang Ronghui, Fang Kuanda, Liu Shenglong, Yang Zhiyong

2021, 35(5): 336-340. doi: 10.3969/j.issn.1007-2993.2021.05.011

[Abstract](192) [FullText HTML] (49) [PDF 842KB](37)

Abstract:
Synchronous grouting with good performance during shield construction is conducive to effective control of ground surface settlement. In order to study the effects of different grout properties on ground settlement, the shield tunnel section of Harbin North Station-Dagengjia Station on Harbin Metro Line 2 was taken as the engineering background. Considering the three characteristics of slurry strength, initial setting time, and stone rate, the finite difference software FLAC3D was used to simulate the ground settlement caused by tunnel excavation, and the influence of different characteristics on ground settlement was analyzed. On this basis, the strength, initial setting time, bleeding rate and consistency characteristics of the slurry were analyzed through the ratio test of different slurries, and the optimal ratio of the slurry was obtained. The results show that: the slurry with high strength, short initial setting time and high stone rate has the best control effect on surface settlement. The amount of cement will increase the strength of the slurry and shorten the initial setting time. However, in water-rich formations, the water setting effect and consistency indicators need to be considered. This study can provide a certain reference and guidance for the improvement of synchronous grouting performance in shield construction.

Test on Dynamic Characteristics of Phosphogypsum Cement Improved Loess

Ai Zhijun

2021, 35(5): 341-346. doi: 10.3969/j.issn.1007-2993.2021.05.012

[Abstract](53) [FullText HTML] (29) [PDF 1192KB](10)

Abstract:
In order to improve the dynamic characteristics of loess subgrade and realize the resource utilization of phosphogypsum, the dynamic triaxial fatigue test of loess improved by phosphogypsum-cement with different proportion was carried out. The test results show that the dynamic characteristics of the loess improved by phosphogypsum-cement are better than that of the plain soil. With the increase of the cement ratio, the greater the dynamic elastic modulus of the loess, the smaller the damping ratio and the smaller the seismic subsidence deformation. Under the low dynamic stress, the loess only experiences the hardening stage in 10000 vibration times. With the increase of the dynamic stress, the loess begins to produce the vibration damage, and with the increase of the cement ratio, the failure mode of loess changes from ductility to brittleness; with the increase of dynamic stress, the dynamic elastic modulus increases at first and then decreases, and the damping ratio decreases at first and then increases. According to the Hardin-Drnevich hyperbolic model, the initial maximum elastic modulus of the loess improved by phosphogypsum-cement increased greatly, but the change of the initial dynamic stress is not obvious. It is proposed to determine the threshold value of the fatigue dynamic stress by the stress-strain curve of equal vibration times. Phosphogypsum cement can effectively improve the threshold value of the fatigue dynamic stress of loess. The research results can provide new methods for the loess subgrade improvement project, and also provide reference for the realization of phosphogypsum resource utilization.

Experimental Study on Horizontal Frost Heave Force of Deep Foundation Pit

Shao Ying

2021, 35(5): 347-351. doi: 10.3969/j.issn.1007-2993.2021.05.013

[Abstract](100) [FullText HTML] (43) [PDF 1323KB](29)

Abstract:
Based on the foundation pit of a subway station in Hohhot City, the extreme cooling process was simulated according to the local climate conditions. Through indoor model experiments and numerical simulation, the distribution characteristics of soil temperature, frost heaving stress and the deformation law of soil and diaphragm wall under the action of freeze-thaw cycle were studied. The results show that: the outer soil temperature gradient is large, and the internal soil temperature is relatively stable and the change is relatively lag, with maximum freezing depth of the foundation pit being 1.1～1.2 m; the horizontal frost heaving force increases at first and then decreases with the depth of the foundation pit, showing a parabola distribution pattern, and the maximum value appears at the depth of about 8～10 m; under the extreme climate conditions, the maximum horizontal frost heaving force reaches 120～140 kN. It has a great influence on the design and construction, and provides reference for deep foundation pit engineering in severe cold area.

2021 Vol. 35, No. 5